U.S. Pat. No. 4,667,158 issued 5/19/87 to Robert W. Redlich for "Linear Position Transducer And Signal Processor" discloses a transducer for measuring straight line motion. Several different embodiments of the transducer are disclosed in that patent, the specification and drawings of which are incorporated herein by reference.
In general, and referring now to FIGS. 1 and 2, the transducer disclosed in U.S. Pat. No. 4,667,158 comprises a hollow tube-like bobbin 1 (formed out of an electrically insulating material such as plastic); a helical coil 2 (formed out of a good electrical conductor such as copper) wound around the outer surface of bobbin 1; a cylindrical rod or tube 3 (formed out of a good electrical conductor having low magnetic permeability such as copper or aluminum), with the rod being axially movable within bobbin 1; an attachment rod 3A connecting rod 3 with the object whose motion is to be measured; an outer shield tube 4 (formed out of a material having both high electrical conductivity and high magnetic permeability such as soft iron or low carbon steel), the shield tube 4 surrounding the remainder of the transducer and confining magnetic flux generated by current flowing through coil 2 to the interior of the transducer and shielding coil 2 from stray fields; and a tubular layer 13 (formed out of a material having high magnetic permeability and low electrical conductivity such as a ferrite powder distributed within a hardened bonding agent), with the tubular layer 13 being disposed between coil 2 and shield 4 and having the effect of reducing the reluctance of this space to a low value.
When attachment rod 3A is connected to an object whose motion is to be measured, and the coil is suitably energized, changes in the position of the object whose motion is to be measured will result in changes in the position of the rod 3 relative to coil 2; this movement of rod 3 relative to coil 2 will, in turn, cause changes in the inductance of coil 2 by means of skin effect, which changes in inductance can be measured to indicate the linear motion of the object.
In U.S. Pat. No. 4,667,158, the tubular layer 13 disposed between coil 2 and outer shield tube 4 is described (at column 6, lines 8-12) as being a ferrite material having a high magnetic permeability but a low electrical conductivity. In addition, it is stated (at column 6, lines 15-17) that ". . . ferrite 13 is preferably a ferrite powder distributed within a hardened bonding agent."
In practice, it has been found that the necessary ferrite powder can be difficult to obtain, particularly in the form desired. In addition, it has been found that fabricating a suitable tubular layer 13 out of the ferrite powder can be more expensive, time-consuming and complex than desired.
For example, in one process currently used to fabricate a transducer of the sort described in U.S. Pat. No. 4,667,158 and employing the ferrite powder tubular layer 13, it is necessary to (a) first prepare a proper mixture of ferrite powder and bonding agent, (b) then fill shield 4 with this mixture, being careful to see that the mixture is evenly distributed within the shield, (c) next spin the loaded shield on a centrifuge under a heat lamp, (d) thereafter allow the loaded shield to cure overnight, (e) next drill out the loaded shield on a lathe to make room for the interior components of the transducer, and (f) then assemble the interior components of the transducer within the loaded shield.
In practice, it has also been found that forming the tubular layer 13 out of ferrite powder as described above can result in performance variations as the temperature of the transducer varies, and can reduce the accuracy of the transducer if not evenly mixed and distributed.